Researchers May Have Solved the Mystery of ‘Ghost Galaxies’

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Ghost GalaxiesAstronomers have been struggling to determine what unseen matter is keeping 'dark' galaxies from pulling apartMichael Sainato12/02/16

Earth is located in the Milky Way, a galaxy estimated to hold between 100 billion and 400 billion stars. But astronomers are beginning to find hundreds of galaxies with hardly any stars at all, and figuring out why they are so dark may help them understand how galaxies form.

In 2015, astronomers found the first batch of these “ghost” galaxies, 47 of them roughly the size of the Milky Way. “If you took the Milky Way but threw away about 99 percent of the stars, that’s what you’d get,” said Roberto Abraham, in a recent interview with Science News. Abraham, an astrophysicist at the University of Toronto, noted, “We’ve gone from none to suddenly over a thousand.”

Astronomers have been struggling to determine what unseen matter is keeping these “dark” galaxies from the gravity of other galaxies pulling them apart. “We speculate that these ‘ultra-diffuse galaxies’ may have lost their gas supply at early times, possibly resulting in very high dark matter fractions,” Abraham and other astronomers noted in their initial study which discovered these types of galaxies.

The technology to find “ultra-diffuse galaxies” has only been recently available. The Dragonfly telescope doesn’t use mirrors, and its lenses are designed to minimize the reflection of light that make these faint galaxies undetectable by other telescopes. Its use on previously studied galaxy clusters have led to the discovery of hundreds of ultra-diffuse galaxies.

How these galaxies formed, and what keeps them intact has remained an enigma for astronomers. If a galaxy is in the process of forming stars and happens to get pulled into a galaxy cluster, the resulting headwind can strip that galaxy of the gas it needs to form stars, according to one proposed theory.

Other astronomers have hypothesized that some galaxies rotate rapidly enough for its stars and gas to spread far out, minimizing its ability to form large numbers of stars.